Brian Brown, NOM’s president, endorsed the candidate in a statement, saying the time has come for social conservatives need to rally around a nominee to defeat President Obama in November.

“We are proud to endorse Mitt Romney for president,” Brown said. “Gov. Romney was an early signer of NOM’s presidential pledge which represents his commitment to the nation to take specific actions as president to preserve and protect marriage as the union of one man and one woman.”

“Now is the time for all people who recognize the importance of marriage to come together to support a true champion, Mitt Romney, against an incumbent who has done virtually everything in his power to undermine the institution of marriage,” Brown continued.

The endorsement comes the day after Romney’s main challenger in the race, Rick Santorum, announced he was suspending his campaign for the presidency, leaving Romney the only candidate with virtually any chance with obtaining the Republican nomination. Like Romney, Santorum was a signer of NOM’s pledge but was known for having even stronger anti-gay views than the former Massachusetts governor.

Brown also took a dig at Obama in the announcement. Among other things, Obama criticized the president for coming out against anti-gay marriage ballot initiatives and appointing individuals who support same-sex marriage as national campaign co-chairs. Obama said he could “evolve” to support marriage equality, but hasn’t yet endorsed marriage rights for gay couples.

Clo Ewing, an Obama campaign spokesperson, highlighted the contrast between Obama and Romney in response to NOM’s endorsement of the Republican candidate.

“President Obama has long believed that gay and lesbian couples deserve the same legal protections as straight couples,” Ewing said. “That’s why he has called for repeal of the so-called ‘Defense of Marriage Act’ and has taken steps to weaken this discriminatory law until the time it can be repealed legislatively. Meanwhile, Mitt Romney, who promised to be to the left of Senator Kennedy on gay rights is now being endorsed by the National Organization for Marriage, an organization committed to limiting the legal protections of the LGBT community.”